1. Field of the Invention
The present invention relates to a storage system and a method for operating the storage system, and more particularly, relates to a technique for efficiently managing differential data to be stored to provide a snapshot function, so that a storage area can be effectively used.
2. Related Art
One of the functions of the storage system is a function of providing a state of a volume in operation (hereinafter referred to as an operation volume) at a certain time point upon request from a user or others. This function (hereinafter referred to as a snapshot function) is implemented in the following way. Specifically, data for reproducing a snapshot of the operation volume at a certain time point is usually backed up in a volume different from the operation volume (hereinafter referred to as a differential volume). Then, upon request from a user, a snapshot at a certain time point is reproduced by using data in the current operation volume and the differential data backed up in the differential volume. For example, see descriptions given in U.S. Pat. Nos. 5,649,152, 7,120,768, and 7,237,076.
To implement the snapshot function in the aforementioned way, management of differential data accumulated in the differential volume is critical. In other words, since the storage system can provide only a limited storage space as a differential volume, it is desirable that the differential data be managed in the differential volume as efficiently as possible.